By the use of germfree animals we plan to continue the determination of the effect of microorganisms on the response of mammals to wholebody ionizing radiation. We are attempting to obviate the histopathology of graft-vs-host disease observed in germfree radiation bone marrow chimeras by pre-treatment of the bone marrow inoculum with anti 0 serum. We are evaluating the immunological competence of these pre-treated allogeneic chimeras by mitogen assays, mixed lymphocyte cultures, cell mediated lympholysis assays, Mishell-Dutton assays, skin grafting, serum protein levels. We are standardizing the in vitro assays by trying to eliminate the variability that results from different samples of fetal calf serum. We are attempting to define the apparent T cell defect observed in the chimeras in terms of an excess of suppressor cell activity. We are also attempting to develop animal models for both acquired and congenital immune deficiency diseases by studying the athymic mice, the asplenic mice, and the hybrids of these two mice in the germfree state. When these immune deficient models are developed, we will try appropriate reconstitution procedures, e.g., fetal thymus, fetal liver, or bone marrow transplantation.